Nowadays an optical add-drop multiplexing (OADM) system having a function of separating and coupling optical signals in wavelength units and a reconfigurable OADM (ROADM) system are being introduced in submarine communication systems using a submarine cable.
FIG. 6 is a block diagram illustrating a general configuration of a submarine communication system 900 into which an OADM system or ROADM system (hereinafter, referred to as “OADM/ROADM system”) is introduced. A node 910 has an OADM/ROADM function.
Sub-bands 1 to 3 indicate wavelength bands of optical signals. A destination of an optical signal to be transmitted from a trunk station 51 whose wavelength band is Sub-band 1 is a trunk station 52. A destination of an optical signal whose wavelength band is Sub-band 2 is a branch station 53. Hereinafter, “an optical signal whose wavelength band is Sub-band 1 (or 2, 3)” will be referred to as “optical signals of Sub-band 1 (or 2, 3)”.
A node 910 transmits an optical signal of Sub-band 2 of an optical signal 13 received from the trunk station 51 to the branch station 53. The node 910 branches an optical signal of Sub-band 1 included in the optical signal 13 received from the trunk station 51 into two and transmits them to the trunk station 52 and the branch station 53. Further, the node 910 removes a dummy signal from an optical signal 15 received from the branch station 53. The node 910 then transmits an optical signal 14 obtained by coupling an optical signal of Sub-band 3 received from the branch station 53 and an optical signal of Sub-band 1 received from the trunk station 51 to the trunk station 52. Optical submarine repeaters which are not illustrated are installed at some midpoints of submarine cables between the node 910 and the trunk stations 51 and 52, and the branch station 53.
In FIG. 6, since an optical signal addressed to the branch station 53 is only an optical signal of Sub-band 2, an optical signal of Sub-band 1 addressed to the trunk station 52 need not be transmitted to the branch station 53. However, by transmitting the optical signal of Sub-band 1 also to the branch station 53, it is possible to prevent an input power to an optical submarine repeater installed between the node 910 and the branch station 53 from decreasing significantly compared with the optical signal 13. As a result, it is possible to operate an optical submarine repeater used in a submarine communication system 900 within a predetermined rated range common to the submarine communication system 900 without significantly lowering an input power. For the same reason, the branch station 53 transmits a dummy signal 5 in addition to the optical signal of Sub-band 3.
FIG. 7 is a diagram illustrating wavelength bands of the optical signals 13 to 15 transmitted and received at the node 910. In FIG. 7, the wavelength band of the optical signal 13 is divided into two wavelength bands, the optical signal 1 of Sub-band 1 and the optical signal 2 of Sub-band 2. The wavelength band of the optical signal 14 is divided into two wavelength bands of the optical signal 1 of Sub-band 1 and the optical signal 3 of Sub-band 3. Further, the wavelength band of the optical signal 15 is divided into two wavelength bands of the dummy signal and the optical signal 3 of Sub-band 3. The wavelength band of Sub-band 1 and the wavelength band of the dummy signal coincide. The wavelength band of Sub-band 2 and the wavelength band of Sub-band 3 also coincide. The wavelength band of Sub-band 1 and the wavelength band of Sub-band 2 do not overlap. Each of the optical signals 13 to 15 has a channel capable of transmitting at least one carrier (optical carrier wave). Each of the optical signals of Sub-bands 1 to 3 also has a channel capable of transmitting at least one carrier. Each carrier is wavelength-multiplexed and transmitted as any of optical signals 13 to 15.
The frequency of an optical signal used as a carrier is defined, for example, by ITU-T Recommendation G. 694.1. ITU-T stands for The International Telecommunication Union Telecommunication Standardization Sector. In this Recommendation, a frequency group of carriers used in wavelength division multiplexing (WDM) used in an OADM/ROADM system is defined as a frequency grid. In the frequency grid, frequencies of the carriers are arranged at constant intervals.
FIG. 8 is a diagram illustrating a more detailed configuration of the submarine communication system 900 illustrated in FIG. 6. The submarine communication system 900 includes the trunk stations 51 and 52, the branch station 53 and the node 910. The trunk stations 51 and 52 and the branch station 53 transmit and receive the optical signals 1 to 3 and the dummy signal illustrated in FIG. 7 via the node 910. The node 910 includes couplers 6 and 12, wavelength filters 7 and 8. The trunk stations 51 and 52 and the branch station 53 and the node 910 are connected by a submarine cable including an optical submarine repeater 54. In FIG. 8, the optical signals 1 and 2 transmitted from the trunk station 51 correspond to the optical signal 13 in FIG. 6. Likewise, the optical signals 1 and 3 received at the trunk station 52 correspond to the optical signal 14 of FIG. 6. The optical signal 3 and the dummy signal 5 transmitted from the branch station 53 correspond to the optical signal 15 in FIG. 6.
The trunk station 51 transmits the optical signal 1 of Sub-band 1 and the optical signal 2 of Sub-band 2. In a coupler 6 of the node 910, the optical signals 1 and 2 are branched into two. One of the two-branched optical signals is transmitted to the branch station 53. The other of the optical signals two-branched by the coupler 6 passes through a wavelength filter 7 and a coupler 12, and is transmitted to the trunk station 52. The wavelength filter 7 transmits the optical signal of the wavelength band of Sub-band 1 and blocks the optical signal of the wavelength band of Sub-band 2. The wavelength filter 8 transmits the optical signal 3 and blocks the dummy signal 5.
As described above, in order to operate the optical submarine repeater 54 installed on a transmission path from the node 910 to the branch station 53 within a rated range, the optical signal 1 is also transmitted from the node 910 to the branch station 53 in addition to the optical signal 2. As a result, the branch station 53 receives a target optical signal 2, while also receiving the optical signal 1 to be transmitted to the trunk station 52.
In connection with the present invention, PTL 1 describes a technique for preventing an unneeded optical signal from being received by a terminal device by blocking an unneeded optical signal by a sub-band by using a cutoff filter arranged in a transmission path. PTL 2 describes an OADM device including a wavelength selectable switch (WSS) capable of shifting the center frequency.